The present invention is directed to an Optical Time-Domain Reflectometer ("OTDR"). More particularly, the present invention is directed to an OTDR that provides inservice measurement capabilities for an optical cable.
An OTDR is a fiber-optic measurement instrument. An OTDR operates by sending a short light pulse down a fiber and monitoring the small fraction of the light scattered or reflected back to it. The reflected light can be used to determine if there is a fault in the fiber.
FIG. 1 is a block diagram of a typical optical transmission system. The transmission system includes an outgoing optical fiber 10 and an incoming optical fiber 12. The fibers 10 and 12 require amplifiers 15-20 because the optical signal attenuates over long distances. However, the typical amplifier includes an optical isolator (not shown) that prevents optical signals from traveling backwards.
An OTDR cannot practically be used with the optical transmission system shown in FIG. 1 because reflected light is blocked by the optical isolators in the amplifiers 15-20. If an OTDR sends a light pulse on the outgoing optical fiber 10 at point 2, no light reflected in the fiber 10 past the amplifier 15 can be returned to the OTDR. Therefore, only a limited section of the fiber 10 up to amplifier 15 can be measured by the OTDR.
A repeater is a unit that includes both an outgoing amplifier and an incoming amplifier. In FIG. 1, a repeater 21 includes amplifiers 15 and 20. Some repeaters include a path between the amplifiers. Prior art measuring devices utilize this path to send light pulses on an input fiber 10 and receive the reflections on the output fiber 12. However, these prior art devices can only be used when the optical transmission system is out-of-service because the light pulses interfere with the traffic signal.
Based on the foregoing, there is a need for an OTDR that can provide some measuring capabilities for an optical transmission system without requiring the transmission system to be out-of-service.